In a liquid crystal display device, which is one type of display device, the liquid crystal display panel is non-luminescent, and therefore a separate lighting device is provided to the back face side (the side opposite from the display face side of the liquid crystal display panel) of the liquid crystal display panel. During display operation, the lighting device lights the back face of the liquid crystal display panel, whereby an image is displayed on the display face of the liquid crystal display panel. A lighting device outfitted to a liquid crystal display device is termed a backlight unit, for example.
It is known to use a cold cathode fluorescent lamp in which mercury or xenon is sealed in the interior of a fluorescent tube, as a light source in a backlight unit. However, cold cathode fluorescent lamps, when employed as light sources in backlight units, entail some inconvenience in that emission luminance and service life are not sufficient, and furthermore in that it is difficult to achieve balanced emission due reduced luminance on the low-voltage end.
To eliminate such inconvenience, there has been proposed a backlight unit having light-emitting diodes (LEDs) as the light source, instead of a cold cathode fluorescent lamp. When LEDs are the light source used in a backlight unit, the aforedescribed inconvenience is eliminated, and in addition, lower power consumption is readily attained, so reduced environmental impact can be achieved as well.
Backlight units are generally classified as being of either of two formats, direct or edge lit.
In a backlight unit of direct format, the light source is disposed in the area directly below the liquid crystal display panel (in an area opposing the back face of the liquid crystal display panel) (see Patent Document 1 for example). In a backlight unit of edge lit format, on the other hand, a light guide panel is disposed in the area directly below the liquid crystal display panel, and a light source is disposed in opposition to a predetermined side edge face of the light guide panel.
Following is a brief description of an example of a configuration for a conventional direct-format backlight device employing LEDs as the light source, made with reference to FIG. 7. FIG. 7 shows an enlarged cross sectional view of part of a conventional backlight unit.
As shown in FIG. 7, in the conventional backlight unit, a plurality of LEDs 101 serving as the light source are mounted on a mounting surface 102a of an LED substrate 102, thereby modularizing the plurality of LEDs 101. The modularized plurality of LEDs 101 are then disposed in an area directly below a liquid crystal display panel (not illustrated).
An optical sheet 103, such as a diffusing sheet, prism sheet, or the like, is disposed in an area a predetermined distance away towards the liquid crystal display panel side from the mounting surface 102a of the LED substrate 102. In so doing, when LED light is emitted by the LEDs 101, the optical sheet 103 diffuses or collects the LED light.
Normally, in order to minimize absorption of light at the mounting surface 102a of the LED substrate 102, a reflecting sheet 104 is disposed over the mounting surface 102a of the LED substrate 102, so that the LED light emitted by the LEDs 101 is reflected by the reflecting sheet 104. A plurality of aperture sections 104a are formed in the reflecting sheet 104, and the LEDs 101 are exposed (protrude) through the aperture sections 104a in the reflecting sheet 104. Specifically, while the reflecting sheet 104 covers the mounting surface 102a of the LED substrate 102, the light-emitting faces of the LEDs 101 are not covered. Therefore, when LED light is emitted by the LEDs 101, the LED light is not blocked by the reflecting sheet 104.